Kovalev improved to 24-0-1 with his 24th knockout, ending the bout with a powerful left jab to Agnew's liver. Agnew (26-1) dropped to one knee in pain and remained there for several minutes after referee Samuel Viruet completed his 10-count at 58 seconds of the seventh.

Kovalev had two other knockdowns in the scheduled 12-round fight. He sent Agnew to the canvas with a left hook to the jaw in the second and dropped him again with a body shot in the sixth.

Kovalev did have to overcome some adversity, suffering cuts above both eyes from a head butt in the fourth and an elbow in the sixth.

"I tried boxing because he has good defense," Kovalev said. "Then I went to the body. I saw how hurt he was."

Agnew spent most of the fight on the ropes, content to absorb Kovalev's punches before launching a counter attack.

Occasionally, he was successful, catching Kovalev with some left hooks and body shots, but eventually was overwhelmed.

"He had an uncomfortable style," Kovalev said. "He has an uncomfortable style. He's not stupid. He kept a good distance. He was patient and I understood he was trying to tire me out, but I was ready."

Kovalev, a native of Russia now living in Fort Lauderdale, Fla., was making his second defense of the championship he won with a fourth-round TKO over Nathan Cleverly in Wales on Aug. 17, 2013. In his most recent outing, Kovalev stopped Ismayl Sillah in the second round in Canada on Nov. 30, 2013.

Saturday's victory was his 11th straight win via knockout or TKO. Because of his power, the 30-year-old is regarded as one of boxing's rising stars. Kovalev entered the fight with the second-highest knockout percentage among current world champions, behind only middleweight champ Gennady Golovkin.

Skeptics wondered how focused he would be for the Agnew fight after a potential unification bout against WBC light-heavyweight champ Adonis Stevenson fell apart last week when Stevenson signed with HBO rival Showtime. Kovalev is under contract to HBO.

"I don't want to speak on Adonis Stevenson," Kovalev said.

Agnew, a 27-year-old native of Chicago now living in Houston, entered the fight as a considerable underdog. Despite his impressive record, he had not fought anyone near Kovalev's level in his first 13 fights. He earned his biggest win in his last fight, a 12-round decision over former middleweight contender Yusaf Mack 11 months ago.